In a small-scale sheep flock located in the State of Mexico with eight ewes and one stud, four ewes had clinical signs of endemic goiter, two aborted fetuses were used for a diagnostic study. In fetuses, thyroid gland samples were used for histopathological evaluation, and liver, heart, kidney and skeletal muscle samples were used to measure selenium. In the ewes the blood serum profile of thyroid hormones was measured by ELISA test. In fetuses there was an increment of thyroid gland size (6-8 cm), firm consistency and red intense color, as well as scarce fleece and subcutaneous edema; there were hyperplasia and hypertrophy of follicular cells of thyroid, with cube-like aspect, diffuse expanded, irregular shape and distension with presence of vacuoles and abundant eosinophilic colloid. The selenium concentrations in liver and kidney of fetuses were low (0.25 and 0.90 ppm). In ewes, the thyroxin (T4) and triiodothyronine (T3) contents were deficient (0.48±0.08 and 4.14±0.29 µg dL-1) and the ratio (T4:T3) was very broad (1:8.6 µg dL-1). It is concluded that the lambs suffered from colloidal goiter; thyroid hormones concentrations in ewes were low and confirmed a thyroid metabolism altered; additionally goiter may be aggravated by selenium deficiency.